For sale we have a mini version of the REUK basic solar water heating pump controller (available here: Solar Pump Controller Circuit in the REUK Shop). This controller is supplied with a pair of (non-waterproof) temperature sensors soldered to 100mm long leads.
When used as a solar water heating pump controller, the first sensor is fitted to the outlet pipe of the solar heating panel, and the second sensor under the insulation on the side of the hot water tank. For any other differential thermostat applications – e.g. turning on a fan to drive warm air from a sunny conservatory into the rest of the house, the first sensor is to be located at the heat source, and the second sensor at the location to be heated.
Whenever the first sensor is measured to be hotter than the second sensor, the output turns on – an output rated to supply up to 2 Amps. This output can be used to directly power a fan or pump rated below around 20W @ 12V, or connected across the coil of a relay if higher currents and/or other voltages are to be switched. The output remains on until the measured temperature difference between the two sensors has fallen by approximately 1 degree Celcius.
If you need a controller which can switch a pump rated at more than 2 Amps, please contact email@example.com as we can modify this differential thermostat to switch the higher current for a small surcharge. For current ratings over 3 Amps we recommend you consider our Solar Water Heating Pump Controller with Relay.
This circuit board is just 45 x 45 x 15mm in size (1.75 x 1.75 x 0.6 inches).
The pump controller requires a constant 12VDC power supply – i.e. it cannot be directly connected to a PV solar panel – it must be connected to a 12V battery or a 12VDC plug in mains power supply (100mA-1A rated).
Note that the leads to the sensors can be extended by up to 10 metres, but the leads to the solar panel sensor should be longer than those to the hot water tank for best results.
Details of the design on which this controller is loosely* based are provided here: Simple Solar Pump Controller together with information on some of the other solar pump controllers and differential thermostats which we produce.
* our version does not have the two 10K potentiometers for calibrating the sensors for simplicity. The sensors read accurately to within 1 degree of each other even without it.